1. Field of the Invention
The present invention relates generally to the technical field of precision casting technique, and more particularly to a casting method using combined 3D printed shell mold and the combined shell mold used in the method.
2. Description of Related Art
The present precision casting technique mostly uses dewaxing casting, the manufacturing technique is:
(1) The drawings are made according to the product to be processed, and the corresponding mold is made according to the drawings.
(2) The wax matrix is made by using the mold and injection molding, and the wax matrix is corrected.
(3) Several wax modules are assembled to form a wax tree, so as to cast multiple products at a time to increase the working efficiency.
(4) The wax tree is treated by gum dipping, so that a layer of paste adheres to the surface of wax tree. When the paste is dry, the gum dipping is performed many times to guarantee the thickness of the gum dipped shell mold. Generally, the gum dipping technique is repeated 5-6 times, the thickness of shell mold is 5-7 mm.
(5) When the gum dipping technique is completed, the steam dewaxing is used to let the wax matrix flow out of the shell mold, so as to separate the wax matrix from the shell mold.
(6) When dewaxing is completed, the shell mold shall be sintered, the paste forming the shell mold is sintered and cured, and the residual wax matrix material is burnt up.
(7) The sintered shell mold is used as casting mold cavity, the molten raw material (e.g. molten metal, molten glass, etc.) is injected into it, and molded in the shell mold. Finally, the whole shell mold is taken out and broken to obtain the molded product.
(8) The molded product is treated by sand removal and ball blast to obtain the finished product.
Therefore, the present dewaxing casting technique is very complex, and the dewaxing, sintering and sand removal techniques result in contamination. Meanwhile the wax matrix manufacturing process is very complex, manual “tree formation” is required, all of these lead to low production efficiency.
On the other hand, the 3D printing technique is developing rapidly, and it has been used in many fields. The present 3D printing is a rapid forming technique in nature, the working process is that a model is built by using computer modeling software, and then the built 3D model is “divided” into sections layer by layer, i.e. slices, so as to guide the printer to print layer by layer, and the thin layers are stacked up till a solid object is molded. The major difference between the multifunctional 3D printer and traditional printers is that its “ink” is generally hot-melt adhesive string, waxy and powdered materials.
This inventor imported 3D printing technique into the field of dewaxing casting through continuous research and experiments, and proposed a casting method using 3D printing to make shell mold, see Chinese patent application number 201510797150.4. The 3D printed shell mold scheme used by the technical proposal is integrated molding, when the drawing is designed by computer, the 3D printer prints out the whole shell mold at a time. As this scheme is used for precision casting, the 3D printed shell mold scheme mainly uses glue jet cementing or laser sintering of fireproof powdered material, e.g. ceramics, in short, the powder is laid layer by layer on the printing platform of 3D printer, the thickness of each layer of powder depends on the required accuracy of the final product. Afterwards, the manipulator driven “printing head” performs gel spray or laser sintering along the preset pattern, and then the powder is laid again. The printed product is formed after the “printing” layer by layer. In subsequent research process, this inventor found that the technical proposal still has some deficiencies, described below:
1. The powder cementing layer by layer or sinter curing molding is used, when the printing is completed, the uncured powdered material shall be cleaned out. However, in the precision casting process, the cast usually has no regular contour, and in order to cast multiple casts at one time, the shell mold for casting is usually “tree-like” shell mold, comprising a primary runner and several pattern dies for molding the required cast. The pattern dies are tree shaped and located on the side of primary runner, so that the channel formed between the runner in primary runner and the pattern die cavity is very zigzag. If the integrated “print” molding is used, it is difficult to remove the residual powder from the pattern die cavity, so that the final cast has deformity or surface asperity, forming a defective.
2. At present, the whole shell mold is molded by “printing” at a time, the 3D printing technique is very difficult. If a part of the printed shell mold is defective, the whole shell mold will be rejected, that is a serious waste; and the product with more complex structure has higher probability of printing defects. In order to reduce the printing defects, the only way is to reduce the structural complexity of the printed product, but the range of 3D printing is limited.
3. The 3D printing technique has progressed significantly, but the printing efficiency is still low, especially for the products of complex construction, the defects are likely to occur in the print run, and the printing efficiency is low, the efficiency of printing acceptable shell molds is very low.
As mentioned above, the present invention proposes the following technical proposal.